The invention relates to a method of rendering the cable core of a telecommunication cable longitudinally water-tight, a method according to which a sealing material is applied in plugs and at regular distances in and around the cable core consisting of stranded conductors and moved at a constant speed, by means of an injection head, which can be displaced intermittently and synchronously with the movement of the cable core in the longitudinal direction of the cable core.
For making a cable longitudinally water-tight, the hollow spaces in the cable core are divided longitudinally of the core into watertight compartments of the same length by plugs of sealing material slightly adhering both to the conductors of the cable core as well as to the sheath and/or envelope surrounding the cable core. The division of the hollow spaces in the cable core into water-tight compartments serves to prevent, in the case of damage of the cable sheath, moisture which may penetrate into the cable core from migrating further along the conductors in the longitudinal direction of the cable and from spreading throughout the cable. If penetrated water is not prevented from spreading, the electrical properties of the cable, such as capacitance and cross-talk, can be considerably reduced. Furthermore, the penetrated water can attach the individual conductors electrolytically via small holes in the insulation referred to as pin-holes. Moreover, there is a risk that water which has penetrated as far as the connection sleeves may cause short circuits between individual transmission networks.
For the sealing material a rubber-like mass known from U.S. Pat. No. 4,451,692 may be used, which during injection under pressure is liquid and after elimination of the pressure is viscous, in other words, has a high yieldpoint stress and a comparatively low viscosity and which cures in due course.
A method of the kind set forth is known from U.S. Pat. No. 4,397,624. In this method, the sealing material is fed from a pressure vessel and pressed via an annular pressure gap radially into the cable core. Due to the fact that the sealing material is only at a comparatively low pressure, the method is limited to cable cores having a diameter of about 25 mm at most. The comparatively low filling speed of the sealing material of about 70 m/sec results in a comparatively long filling time of about 10 s per cycle. In practice, the sealing plugs have a length of 20 to 30 cm. This method is further limited to cable cores having about 200 conductors at most and to cores the individual conductors of which have a diameter not exceeding about 1,8 mm. By means of this method, a maximum production speed, i.e. travelling speed, of the cable core of 0.2 to 0.2 m/s can be attained, dependent upon the diameter of the cable core and upon the number of conductors.
Within the said limits, the known method is satisfactory in practice. Due to the fact, however, that in this method the sealing material is pressed into the cable core along its entire circumference, there is moreover a risk, especially with regard to cable cores of larger diameters, of the conductors being compressed and the core being constricted so that the sealing mass cannot penetrate into the heart of the cable core.